Storage systems that use optical disks as the storage media lose time when access to the recorded data waits upon the optical focusing servomechanism to acquire focus onto the recording surface of the spinning optical disk.
A patent of interest for its teachings relevant to the state of the art is U.S. Pat. No. 4,497,047 entitled "Optical Disk Player with Focus Control During Search Mode" by K. Fujiie et al. The focusing control system of the patent responds to the need of quickly acquiring focus when shifting transversely across a plurality of tracks to a new read position. The system reduces the gain of the focusing servo but maintains servo control at a defocused level such that the servo control system does not have to commence the acquisition of focus at the new read position from a start or reference position. Additionally, if a sharp focus was to be maintained as the objective lens moved in the traverse manner across the recording tracks it would be subjected to vibratory movements as the servo mechanism attempted to lock onto the different refelective patterns that passed through the field of view of the lens as it was moved to the new read position.
Another patent of interest is U.S. Pat. No. 4,512,003 entitled "Control Circuitry for Preliminary Focusing of Light on a Record Medium Surface in an Optical Reproducing Apparatus" by S. Kimura et al. The apparatus of the patent uses a preliminary focusing signal which is derived from a signal generator such that a first polarity signal, that varies rapidly at one polarity and then varies slowly towards a second predetermined level of opposite polarity, creates a zero crossover point corresponding to a preliminary focus point for the servo system. Once preliminary focusing is achieved, the system shifts into the second level of focus utilizing a focus error signal generated by the focusing servo.
A third patent of interest is U.S. Pat. No. 4,677,605 entitled "Focus Acquisition and Maintenance for Optical Disk System" by M. Abed. In the system of that patent there is also disclosed a two-level focusing technique which utilizes the asymmetrical properties of a light beam reflected from the surface of the recording disk onto a quad detector wherein the electrical signals resulting therefrom are compared against a threshold value as the objective lens is driven towards a focusing position and when the electrical signals'value compares to a known threshold value, indicating a coarse focus, the system is switched to a fine focus servo configuration so that the fine focus can be achieved. Additionally, in column 8, lines 41-49, it is mentioned that the method of the invention is used to avoid a false focus on the overcoat layer of the optical disk. This is accomplished by having the objective lens relatively close to the disk or focused behind the disk and as the objective lens is ramped away from the disk the first legitimate focus appears on the recording material and not on the overcoat layer. In this manner a false focus is averted.
A fourth patent of interest is U.S. Pat. No. 4,396,959 entitiled "Data Transducer Position Control System for Rotating Disk Data Storage Equipment" by J. N. Harrison et al. The system of that patent uses wide-driving pulses to quickly move an optical head across many tracks when the track to be focused on is located a considerable distance from the optical head's initial position.